The present invention relates in general to a vibration-damping structure, and more particularly to an elastic block structure such as an engine mount, excellent in its damping characteristics for both high- and low-frequency vibrations.
In the art of a vibration-damping elastic or rubber block structure used as an engine or body mount in an automotive vehicle, there has been known a structure including a rubber block interposed between two metal supports. Such elastic structure generally employs a rubber material having a low dynamic spring rate (constant) for improved noise isolation for high-frequency vibrarions, and consequently has a comparatively low loss factor (loss tangent) and damping coefficient. As a result, the known elastic structure suffers a low vibration-damping capability at low frequencies of vibrations, and is not completely satisfactory in meeting the properties required of an elastic vibration damping rubber mount.
On the other hand, another type of elastic vibration damper has been proposed, which utilizes resiliency of a rubber or elastic material and flow resistance of a fluid. This damper is known as a fluid-filled or fluid-containing damper, a typical example of such type of damper is disclosed in British Patent Specification No. 811748 wherein a cavity within a rubber mass is closed by a diaphragm to form on one side thereof a first fluid chamber which is filled with hydraulic fluid. This chamber is placed in fluid communication with a second chamber provided on the other side of the diaphragm, through a central orifice of suitable length and diameter formed in the diaphragm. In this arrangement, the fluid flowing from one chamber to the other will permit a high loss factor at low frequencies of vibrations.
Such fluid-filled damper, however, tends to have a difficulty in flow of the fluid through the orifice upon application of high-frequency vibrations, and suffers a problem of increased fluid pressure within the first chamber which necessarily results in increased dynamic spring rate, making it undesirable to use such damper as an elastic engine mount or the like. This increase in the dynamic spring rate for the high-frequency vibrations is a result of efforts to attempt to increase a variation in the volume in the first chamber for increasing a loss factor of the damper for low-frequency vibrations.
It is noted here that the dynamic properties of a fluid-filled elastic mount is a vectorial sum of the dynamic property of a rubber material and the dynamic property of the working fluid, i.e., the dynamic spring rate (Kd) of the elastic mount as a whole is expressed by the following equation: EQU Kd=Kd1+Kd2
where:
Kd1: Dynamic spring rate of rubber material PA1 Kd2: Dynamic spring rate of fluid PA1 Kd1(100): Kd of rubber block at 100 Hz PA1 Kd2(100): Kd of fluid at 100 Hz PA1 Kd1(100): Dynamic spring rate of rubber block PA1 Kd2(100): Dynamic spring rate of fluid PA1 Ks: Static spring rate of the rubber block (=static spring rate of the elastic mount)
Since the dynamic spring rate (Kd) of the fluid is determined by the fluid pressure, the dynamic spring rate (Kd) of the elastic mount is increased as the fluid pressure is increased, i.e., as the frequency level of the vibrations is elevated. Stated the other way, the fluid pressure is proportional to a variation in the volume of the fluid chamber upon application of the vibrations, and the loss factor is also proportional to the volume variation of the fluid chamber upon application of the vibrations.
In conclusion, the fluid-filled elastic mount discussed above is designed for increased variation in the volume of the fluid chamber for the purpose of obtaining a high loss factor for low-frequency vibrations, whereby the dynamic spring rate is necessarily elevated at high frequencies of vibrations.
To avoid an increase in the fluid pressure within the fluid-filled elastic mount at high frequencies, there is proposed in U.S. Pat. No. 4,159,091 a resilient damper device wherein a partition wall dividing a fluid chamber is provided with a movable part or movable plate which is freely movable in a direction perpendicular to the partition wall upon application of small-amplitude vibrations of high frequencies, thereby preventing a rise in the fluid pressure until the vibration frequency is elevated beyond a given level. In this fluid-filled mount, large-amplitude vibrations of low frequencies will cause the movable part or plate to be slightly moved until it is blocked by a stop. Subsequently, the fluid flows from one chamber to the other through an orifice formed in the movable part or plate, whereby a high loss factor is obtained.
However, the slight movement of the movable part or plate tends to give the elastic mount a lower loss factor as compared with that of a fluid-filled elastaic mount without a movable part or plate on the partition wall. Therefore, the elastic mount as disclosed in the above-identified U.S. Pat. is designed for increased variation in the volume of the fluid chamber and for increased fluid volume to obtain a higher loss factor (which leads to a high dynamic spring rate). In other words, the movable part or plate is used to lower the dynamic spring rate which is higher in the above design.
However, the fluid-filled elastic mount provided with a movable plate has a problem that a fluid mass or column supporting the movable plate undergoes a phenomenon of resonance oscillation at vibration frequencies higher than 50 Hz. The resonance frequency is determined by the diameter and the length of the fluid column, the configuration of a rubber or elastic block used, the static spring rate (Ks) of the elastic mount, etc. When such resonance occurs, the fluid pressure in the chamber varies to a great extent, and the dynamic spring rate which has been held at a low level is elevated. As a result, if the elastic mount is used as an engine mount, the transfer characteristics of vibrations, i.e., a force [Kd.times.(amplitude of vibration)] to be transferred from an engine to a support structure of a vehicle, is increased and the vibrations coming from the engine are easily transferred to the support structure, whereby the noise isolation and ride comfort characteristics are degraded. In addition, the resonance oscillation will cause a large variation in the transfer of vibrations, which is not desirable as a characteristic of a vibration-damping structure, because a sudden change of vibrational transfer will result in variation in the level of sounds transferred to a passengers' compartment of the vehicle. Due to such variation in the sound level, the sounds are heard as noises unpleasant to the passengers. Hence, the engine mount is required to permit a smooth variation in the transfer of vibrations, i.e., to provide excellent transfer characteristics.
An improvement in a fluid-filled elastic mount having a movable plate as discussed above, is disclosed in German Offenlegungsschrift No. 3019337, wherein a ratio (L/D) of a length (L) of an orifice to a diameter (D) thereof is held within a range of 4-80 which is comparatively higher than that of an orifice of the prior counterpart, and the orifice is provided not in the movable plate, but in a member separate from the movable plate. While this improved type of fluid-filled elastic mount exhibits low Kd (dynamic spring rate) characteristics at low frequencies due to provision of the movable plate, it also suffers a resonance oscillation of the fluid mass supporting the movable plate upon application of vibrations of higher than 50 Hz, thereby creating problems of abrupt change in its transfer characteristics and increased value of Kd, that is, low transfer characteristics and low dynamic properties, and consequently low noise isolation, as experienced in the elastic mount previously described.